The present invention relates to damping of oscillations of the first bending mode of a wind turbine by means of oscillation damping means comprising one or more containers partly filled with a liquid for damping oscillations of the first natural bending frequency of the wind turbine.
In particular, the invention relates to damping means comprising a plurality of box shaped containers having a square cross section and being partly filled with liquid so as to provide a unidirectional damping of a magnitude being equivalent to a logarithmic decrement of oscillations of the first natural bending frequency of the wind turbine of at least 4-8%, the total mass of the liquid contained within the containers being equal to 1.2-1.8% of the mass of the nacelle. In case of off-shore positioned wind turbines, the combined excitation of the wind and sea waves requires more damping, the damping is preferably equivalent to a logarithmic decrement of 10-15%, which may be obtained with a total mass of the liquid being equal to 2-4% of the mass of the nacelle.
Furthermore, the invention relates to damping of oscillation of the second bending mode of a wind turbine and to the combined damping of oscillations of the first as well as the second bending mode of the wind turbine.
All tall buildings and constructions are excited by the wind in the full frequency spectrum including the natural bending frequencies of the building. Due to a positive proportionality between the horizontal forces on the building and the wind speed, the wind has a dampening effect on oscillations of the first natural bending frequency for buildings of a slender cylinder shape, such as chimneys, and for traditional wind turbines having such a characteristic of the horizontal forces.
Modem wind turbine has a characteristic of the horizontal forces that has a plateau or even has a negative proportionality to the wind speed above a certain limit. The latter characteristic is advantageous with respect to the power production of the wind turbine, but the construction becomes less stable because the wind will amplify oscillations of the wind turbine when the wind speed is above the limit, typically about 10 m/s. The negative proportionality has the consequence that the horizontal forces on the wind turbine increase when the top of the wind turbine swings away from the wind and the relative wind speed at the top decreases and that the horizontal forces decrease when the top wings towards the wind and the relative wind speed increases.
It is known from the prior art to dampen oscillations of the first natural bending frequency of wind turbines by means of mechanical damping means comprising one or more masses, springs and dampers. Such solutions to the oscillation problem are expensive for wind turbines since relatively large masses of the damping means must be used because the equivalent swinging mass in the first natural bending mode is very large since the nacelle comprising a power transmission system is arranged at the top of a slender tower.
It is known from EP 0 648 906 A1 and EP 0 686 733 A1 to dampen oscillation of slender buildings such as chimneys, masts etc. by means of containers being partly filled with liquid and being tuned to dampen the natural frequency of the building. The containers employed are of a quasi-symmetrical cross section such as circular, quadratic or triangular so as to exhibit quasi-symmetrical damping characteristics which are advantageous for buildings having a symmetrical oscillation behaviour. Similar damping of slender buildings with quasi-symmetrical containers is known from e.g. U.S. Pat. No. 4,873,798, U.S. Pat. No. 4,783,937, U.S. Pat. No. 4,924,639, U.S. Pat. No. 4,875.313 and U.S. Pat. No. 4,922 671. The principles of the applied containers may also be used for the present invention, but the damping of the natural frequency of a wind turbine is very different from damping the oscillations of a slender building.
Contrary to a slender building such as a chimney, the equivalent oscillating mass of the first natural bending mode is equal to 85-90% of the total mass of a wind turbine, whereas the equivalent mass of a chimney is about 10-15% of the total mass. This difference is due to the concentration of mass in the nacelle of the wind turbine which typically contribute with about 85% of the oscillating mass. Furthermore, the rotor arranged at the top of the wind turbine exposes the wind turbine to heavy wind forces at the top as well as to periodic excitation with the frequency of rotation of the rotor as well as three time (for a three blade wind turbine) the frequency of rotation. The formation of a von Karman vortex alley which is well known for slender building is on the other hand negligible for wind turbines.
Oscillations of wind turbines has always existed but it seems that the problem is getting more pronounced with the development of the new generation of very large wind turbines. At high wind speeds ( greater than 20 m/s) and some yaw errors calculations show that the combination of the structural damping of the tower and the aerodynamic damping of the rotor and nacelle is too small. More energy is going into the system from the wind than out of it. The result is that the oscillations are getting out of control which will lead to failure.
A xe2x80x98softxe2x80x99 generator, i.e. a generator with a high slip, can damp the oscillations, but such a generator is much more expensive and larger than a generator with lower slip. It is therefore avoided as a solution to the problem.
The oscillations can be reduced by changing the natural frequency of the tower, but it does not solve the main problem that more energy is going in than out of the system. The best and cheapest solution is to install damping means in the wind turbine. The damper can be placed where the movements occurs, but as the amplitude of the oscillations is largest in the top of the tower and in the nacelle, the upper part of the wind turbine is the optimal place. The damper may be placed in the tower, in the nacelle or outside.
Damping of oscillations at the wind turbines first bending frequency will in general reduce the fatigue loads on the tower and because of that reduce the necessary amount of steel in the tower.
However, an efficient damping of oscillations leading to a more lean construction of the tower of the wind turbine, especially in combination with higher towers, up to about 120 m, may result in the occurrence of oscillations of the second natural bending frequency of the wind turbine, which again may lead to fatigue loads on the tower. The damping of second order oscillations have not been recognised or addressed previously in the known literature of wind turbines for the reason that the problem has not been relevant for the shorter and more robust structure of wind turbine tower without damping means for damping the frequencies of the first natural bending mode
In order to fulfil an object of the present invention of providing an efficient damping of oscillations of the first natural bending frequency of the wind turbine commercially, it has been found to be advantageous to provide the wind turbine with a damper in which the oscillating mass is a liquid which opens for both a simple construction of the dampers as well as for the use of an inexpensive oscillating mass, such as water.
It is a further object of the present invention to provide damping of oscillations of the second natural bending frequency either in itself or in combination with damping of oscillations of the first natural bending frequency according to the invention.